Acoustic emissions are transient elastic waves generated by the rapid release of energy within a material. During impact of an automotive vehicle with an object, acoustic emissions are generated by plastic deformation, grain boundary slip, crack initiation and crack growth occurring in the metal structure of the vehicle.
Acoustic emission sensing is presently used for the non-destructive testing of large pressure vessels and structures to determine the integrity of the material and of the welds thereof. Acoustic emission impact sensing for actuating armaments is also known.
The acoustic emission signals may be sensed by force transducers such as piezoelectric crystals having a resonant frequency sensitive to surface wave frequencies in the 100,000 kilohertz to 1.0 megahertz range. The surface wave impact on the crystal surface causes the crystal to generate an electric output signal which can be amplified, filtered and digitized. The digital data can then be compared to preprogrammed data in a microprocessor to determine whether the thresholds of certain parameters have been exceeded.
It is known to provide occupant restraints in an automotive vehicle which are actuated upon the occurrence of a predetermined level of vehicle impact. Such restraints may be of the inflatable cushion type, the belt type, or of other types and are electrically or pyrotechnically actuated when a velocity or acceleration responsive sensor indicates the existence of an impact level exceeding a predetermined level.